In the late 19th and early 20th centuries, the design of automotive wheels was based on that of bicycle tires, having thin walls with high pressure and narrow profiles. The advantages were high energy efficiency with low energy loss due to tire flex. Disadvantages were a hard ride and frequent punctures. As time went on, engines became more powerful and fuel became less expensive. Freedom from flats and ride comfort took top priority, leading to wider, lower pressure, thicker tires. These tires involved greater energy loss mainly due to tire flex at and near the area of contact with the road. Recently however, energy conservation has once again become of prime importance. The need for higher energy efficiency in the wheel, among other automotive subsystems, is paramount.
Numerous approaches to increasing the effectiveness of electric and hybrid vehicles have been implemented: shaping the vehicle to reduce aerodynamic drag, larger battery capacity, and regenerative braking are among them. These approaches are effective as far as they go. However, little has been done to remedy a serious source of wasted energy in vehicles: the process of vehicle kinetic energy being converted to heat due to tire flex.
Under smooth road conditions, the ideal energy conserving tire has thin walls and is inflated to high pressure. The tire is supported by the rim which functions as a rigid support. A narrow high pressure bicycle tire is an example. A small amount of tire flex occurs at or near the road contact, leading to reduced energy loss.
Under rough road conditions with large obstructions, shock absorption and cushioning are the major requirements, that only a low pressure, large chambered tire can satisfy. Conventional automotive tires meet these requirements well. These tires rely on one inflated low pressure thick-walled air chamber for operation on both smooth and rough roads. Thus a conventional tire is largely biased towards cushioning on rough roads, while sacrificing the energy saving characteristics of narrow high pressure tires.
Past attempts have been made to design multichambered pneumatic tires with energy-conserving features. Lambe (U.S. Pat. No. 5,109,905) discloses a two-chambered pneumatic tire, with the goal of reducing tire flex and conserving energy. An outer high pressure chamber is intended to simulate a high pressure pneumatic tire. An inner low pressure pneumatic chamber is intended to simulate a conventional low pressure tire with cushioning effect.
However, Lambe's tire would result in at most a small improvement in efficiency over a conventional tire, for two reasons. Without internal restraints in the low pressure chamber to position the outer chamber relative to the hub, a very high outer chamber pressure would be required to stiffen the tread sufficiently to adequately reduce tire flex at and near the road contact. There is nothing present that enables simulation of the rigid support provided to a narrow high pressure tire by its rim. Also, even if the outer chamber were stiffened substantially, without internal restraints in the low pressure chamber the outer chamber would not remain centered on the rotational axis. Thus the sidewalls would flex to about the same extent as with a conventional tire. Much of the outer chamber would move vertically in response to a road obstruction, possibly leading to an actual reduction in efficiency. The outer chamber may have increased stiffness because of the high pressure, but the restoring force profile (i.e. the restoring force as a function of tire deformation) is similar to that of a conventional automotive tire. As a result the Lambe tire cannot simulate a high pressure pneumatic tire on a smooth road as effectively as the present device.
An energy conserving wheel featuring a rigid outer tread suspended over a rim by an elastic band is disclosed by Russell (U.S. Pat. Nos. 6,701,985 and 7,104,297). While this wheel is intended for heavy duty applications, it has three basic problems. The distance from tread to rim is shallow, leading to possible rim damage from road impacts. Second, only a small portion of the elastic band actually stretches to play a direct part in supporting the tread, leading to an inefficient use of material. Third, the elastic band is intended to slide against the perforations of the tread actuator, this occurring constantly on each rotation of the wheel. This may cause wear and failure of the elastic band.
References to multi-chambered tires for the purpose of reducing the effects of punctures occur, for example in Fulsang (U.S. Pat. No. 6,470,935), Bushemi (U.S. Pat. No. 2,572,594), and Murphy (U.S. Pat. No. 580,884). These examples make no reference to energy saving features.
References to hub protectors exist, for example Boiocchi et al (U.S. Pat. No. 7,100,654), French (U.S. Pat. No. 5,885,383), and Pompier (U.S. Pat. No. 4,922,981). These devices serve to reduce damage to the hub and rim after a puncture, but are not designed to function as shock absorbers or suspensions.
The present device features a tire which meets the requirements for rough and smooth roads in such a manner that each of the two requirements comes into play only when required by the specific road condition. Thus each of the two requirements can be met separately and optimally. On a smooth road, internal restraint bands hold the outer tread ring in a position concentric with the hub and axle. On a rough road, that part of the outer ring near the road contact flexes inward toward the hub, bringing the cushioning effect of the low pressure chamber into play. Additional reductions in energy consumption can be gained by incorporating an electric hub motor, which reduces or eliminates the need for the typical drive train implemented between the engine and the conventional wheel.
The previous attempts at producing a heavy duty tire design combining the advantages of energy conservation on smooth roads and cushioning on rough roads have proven inadequate. The present device however, meets these requirements: (1) simulation of a high pressure pneumatic tire on smooth roads, (2) simulation of a cushioning effect of a conventional low pressure tire on rough roads, (3) functioning as a shock absorber and suspension on rough roads, and (4) a non-pneumatic outer tread ring, to prevent punctures.